The Caldwell Objects

telescope. But seeing it takes more effort than one
is likely to invest in a random patch of sky during
a comet sweep. Actually, NGC 6729's tiny glow
escaped the gaze of many great southern-sky
observers, including James Dunlop (who
discovered NGC 6723) and John Herschel. It took
the keen eye of Albert Marth — an "able and
hardworking German astronomer who had
emigrated to England," as Joseph Ashbrook
describes him in his Astronomical Scrapbook — to
find the diminutive nebula. "Marth," Ashbrook
continues, "has never received adequate
recognition for his varied contributions to
astronomy." In 1861 Marth traveled to the
Mediterranean island of Malta with William
Lassell, who set up his great 48-inch reflector
there. Over the course of the next four years,
Lassell and Marth used the telescope to scan the
heavens. According to Ashbrook, the "most
important result of this
survey was the recognition
of spiral structure in a
number of galaxies. Also,
a catalogue of 600
previously unknown
nebulae was compiled [by
Marth]. Most of the actual
observing during this
second Malta expedition
was not done by Lassell
but by his assistant Albert
Marth."
NGC 6729 was the
395th object Marth cataloged.
It was described
as part of a larger, "pretty
faint" complex of nebulosity,
namely NGC
6726-7. NGC 6729 is but a
tiny 1'-long wisp of gas
flowing away from R
Coronae Australis to the
The Caldwell Objects
68
southeast. In photographs it looks like a fanshaped
comet tail. In 1866, Julius Schmidt
(Athens Observatory) first noticed that R
Coronae Australis's brightness varied irregularly.
Generally its magnitude lies between 9.7 and
13.5. The nebula's brightness variations appear to
follow the star's. The nebula also changes form
rapidly — too rapidly, in fact, to represent true
changes in its configuration, since the changes
appear at face value to propagate far more
rapidly than physically possible. John A. Graham
(Carnegie Institution of Washington) and his
colleagues documented this phenomenon in the
February 1987 Publications of the Astronomical
Society of the Pacific. (Not that they were the
first, as the photographs below, taken from
Mount Wilson, attest.) With their CCD images,
obtained over a 23-day period in 1984, Graham's
team found that the reflection nebula's surface
brightness can change
271